Recent improvements in the manufacture of high power solid state amplifiers have given rise to applications in new fields such as microwave cooking, ignition engine efficiency and in medical devices and treatments.
Conventionally, an RF heating apparatus such as a microwave oven generates RF power to be introduced into a cavity using the device known as a magnetron. A magnetron is an oscillator-amplifier that typically provides RF energy only at a single frequency (for example 2.5 GHz).
The efficiency of the heating provided by a microwave oven is dependent upon the proportion of the RF energy introduced into a cavity of the oven that is actually absorbed by the food or beverage being heated. Normally, at least some of the RF energy introduced into the cavity is reflected back to the magnetron, whereby the power efficiency of the heating apparatus is reduced. It is well known that the reflection of the RF energy inside the cavity depends on factors such as the wavelength, phase and amplitude of the RF radiation, the size, shape and cross section of the food or beverage, and the dimensions and shape of the cavity itself.
Accordingly, it is known that one way to optimise the amount of RF energy that is absorbed by the food or beverage being heated is to trim the physical parameters of the RF radiation, to minimise the reflected signal. These parameters include the amplitude, frequency and/or phase of the radiation. Although a magnetron is a relatively cheap component, it does not allow for this kind of trimming. On the other hand, solid state devices may be able to provide trimming since they can enable multi-frequency, multi-phase operation, with multiple paths.
FIG. 1 shows an example of an RF circuit 10 including solid state components that can be used to implement trimming of the kind noted above. The circuit 10 includes a plurality of paths A, B, C. Each path includes a phase locked loop (PLL) 2A, 2B, 2C for producing an RF signal. As shown by the dashed lines, the PLLs 2B and 2C may in some examples be disabled (or simply omitted), such that PLL 2A can be used to provide the RF signal for each path. In such examples, the PLL 2A is thus a common PLL that is shared by each path A, B, C, and each path A, B, C would typically operate at the same frequency (i.e. the operating frequency of the PLL 2A). Where separate PLLs (e.g. 2A, 2B, 2C) are provided for each path, multi-frequency operation may be enabled.
An output of each PLL 2A, 2B, 2C (or, as noted above, the output of a common or shared PLL 2A) is connected to phase shifters 4A, 4B, 4C. The phase shifters 4A, 4B and 4C can be used to apply the phase shifts to the RF signal of each path under the control of a microcontroller 14. Accordingly, the microcontroller 14 may adjust the phase of each path for trimming the RF radiation produced by the system. Note that the micro-controller may also control the PLLs 2A, 2B, 2C to adjust the frequency of the RF signal in each path A, B, C.
The phase shifted signals are then provided to variable gain amplifiers 6A, 6B, 6C and then to power amplifiers 8A, 8B, 8C for subsequent introduction of RF radiation into the cavity of the heating apparatus by respective antennae 12A, 12B, 12C.
When each path works at the same operating frequency, it is important for the phase between the paths to be accurate and not time varying. Typically, this property can only be achieved if one of the paths provides a phase reference (e.g. a reference signal used by PLL 2A) to each of the other paths so that it is possible to provide phase coherent signals to the phase shifters 4A, 4B, 4C on a local and individual basis without changing the global phase coherence.
FIG. 2 illustrates trimming and optimisation of RF signals of the kind described above. In particular, for multiple signal paths, FIG. 2 illustrates that close and accurate control of the phases between the signal paths is important for effective trimming. Optimal heating efficiency in FIG. 2 is obtained at operating frequency Fopt and relative phase φopt (this assumes two paths although, of course, the principal explained here may be extended to three or more paths).
Accurate control of the frequency to trim to Fopt may be relatively easy to implement using PLLs under the control of a microcontroller. However, accurate control of the relative phase is much more difficult, because of the instantaneous nature of the phase: it is a delay between two signals at same frequency. At high frequencies, this delay can be as small as a few picoseconds for a couple of degrees.
U.S. Pat. No. 3,906,361 A describes a system for measuring the phase difference between two signals and having a digital readout. Each of the two signals are fed to Schmitt trigger circuits which convert the sine wave signals to square waves. The two square waves are compared for coincidence in a discriminator, the output thereof being converted by a logic circuit to a form for displaying the digital phase difference of the two signals
U.S. Pat. No. 6,351,153 describes phase detector that detects the phase of two inputs with precision. Common errors due to temperature variations and supply voltage fluctuations are subtracted out. The phase detector and method utilize digital circuitry such as exclusive OR gates and differential amplifiers to perform the accurate phase detection. The inputs and outputs may be attenuated or filtered to produce the desired results.
WO 2013/156060 A1 describes a phase detector providing linear phase information from −180 DEG to +180 DEG. The phase detector comprises a frequency divider, a phase shifter and a mixer circuit. The frequency divider and phase shifter have an output port connected to an input port of the mixer circuit.